This invention relates to a method of inspecting an anisotropic etch in a microstructure, for example, to measure the depth of an anisotropic etch in a monocrystalline material. A microstructure is any small scale structure, such as a microelectronic devices on wafer used in semiconductor fabrication.
Liquid anisotropic etching is used extensively to form holes in semiconductor material. An etchant is chosen such that the etch rate in the vertical direction is greater than the transverse direction. The result is a hole bored into the semiconductor material.
Having formed the hole, it is necessary to know its depth. Many methods of measurement exist. The etch depth can either be measured by contact profilometry, which takes long scanning time, or alternatively by optical interferometry, which requires an expensive and alignment sensitive tool. It can also be done by using the microscope focusing height difference technique. This is time consuming and not reproducible or requires an expensive tool.
What is needed is a quick, reproducible and inexpensive process-integrated method for evaluating the etch depth of an anisotropic etch of a microstructure such as a monocrystalline material.
According to the present invention there is provided a method of evaluating an anisotropic etch in a microstructure, comprising providing a substrate; forming a film on said substrate; forming a series of holes of progressively different area and having specific geometric shapes through said film; performing an anisotropic etch in said microstructure through said holes by relying on different etch rates in different crystal planes under known and reproducible conditions; and inspecting said microstructure through said holes after said anisotropic etch to compare results from holes of different area.
It will be understood that anisotropic etching relates to the difference in etch rate with regard to direction. One way of achieving an anisotropic etch is to rely on the difference in etch rate with respect to different crystal planes lying at an angle to each other. For example, the etch rate is fast in the etch plane {100} but very slow in the {111} planes, which lies at an angle to the {100} plane.
An anisotropic etch typically produces V-shaped grooves, inverted pyramids or more complex shapes. The etchant quickly eats away at the bottom of the hole being etched, which lies in the {100} plane, but only slowly attacks the walls of the hole formed by the {111} planes. As a result, there comes a certain point where the walls of the groove meet, and at this point the etch essentially stops. The invention makes use of the fact that the depth at which this point occurs depends on the initial area of the hole. The larger the area of the hole, the deeper will be the etch. If the substrate is etched through holes of different diameter, the hole where the walls just meet at the bottom of the V-shaped groove permits the etch depth to be calculated knowing the etch rate ratio for the different planes.
The method of the present invention is typically used for determining the etch depth, but the method can also be used for other types of wafer evaluation.
In the preferred embodiment, many fabrication steps are integrated or common to an anisotropic etching fabrication process of microstructures. The first step requires the formation of masking film resistant to the anisotropic etchant on top of the monocrystaline material. Then patterning and selective etching of that film in geometrical shapes of different dimensions (preferably in increasing or decreasing order) is accomplished. The chosen shapes will permit discrimination of the anisotropic etch depth through visual inspection after this etching step.
The visual inspection can be done either by unaided eye, through an optical microscope, an electron microscope, a robotic inspection system or any other inspection method. The visual or the automatic pattern recognition distinction is made between a different geometrical shape when a partially etched pattern is compared to a completely etched pattern or another partially etched pattern. Each shape is preferably identified with a label to help the inspector to determine the corresponding etch depth. The etch depth can be determined by the last etched shape, be it completely or partially etched, and a different partially etched shape. The number of shapes as well as the resolution of the inspection system determines the precision of the method. For determining the label, the etch rate of the slow etching plane relative to the etch rate of the wafer plane must be considered.
This method may or may not require a precise angular alignment of the patterns to the crystal structure depending on the shape used. The angular alignment of the crystal plane to the wafer surface plane should be tight. That alignment is called the front surface off orientation.